The SIR provided L-AMinine-guanido- 13C, "N2; 15mg 'C,'N RNA; 1.2g 13C, "N RNA; 292mg 13C, "N RNA; 320mg "C, "N RNA; 560mg The IRE cis element in eukaryotic mRNA is highly conserved and has a characterized specific regulator protein. Our studies of function and structure, using site-directed mutagenesis and cleavage by shape-selective transition metal complexes as structure probes, indicate a folded structure with possibly unusual tertiary features. Our preliminary NMR spectroscopy shows that the IRE sequence (a 30 mer) forms a single conformer in solution which facilitates the structure analysis. The use of 13C- and 15N-enriched ribonucleotides, uniformly labelled or otherwise, are crucial to the NMR study.